In the development of radiation detectors—for example X-ray detectors for CT systems—an important consideration is to reduce manufacturing costs ever further. The main costs of a CT detector in such cases may be found in the sensor board. This usually contains the direct-converting or indirect-converting sensor material, the evaluation electronics (ASIC and in a few assemblies the photodiode as well) and a carrier material or carrier substrate, which may be needed as the base unit for the structure as a whole and gives the sensor board its mechanical stability.
The surface of ASIC in this case is usually equal to the surface of the overall CT detector. This is the case for example in the technology for integrating detectors, in which the photodiodes and the ASIC form one unit. The advantage of this approach lies in the fact that the line lengths between photodiode and evaluation electronics are kept as a short as possible in order to reduce electronic noise. Furthermore the technology has been developed to make smaller pixels in the detector possible.
With counting technology too the surface of the ASIC is usually equal to the surface of the overall CT detector. A large unknown in the development of the counting technology is in certain parts the influence of the input capacitance of a pixel on the response behavior and in such cases in particular the linearity and noise behavior in combination with the energy resolution of the ASIC. In order to take complexity and risk out of the development, the general aim has been to keep the input capacitances as low as possible and to keep them the same size. This may lead to a basic structure (see FIG. 1), in which the respective input channel of the ASIC is located directly on the sensor-side pad. The line lengths are minimized in such cases and where possible are designed so that the input capacitances are the same for all pixels.
The disadvantage of this highly-integrated approach on the other hand is the price. The ASIC costs are determined as a rule not by the functions contained, but by the surface. In particular with a view to larger sales markets, the aim should thus be to reduce the costs.